In order to protect electrical equipment, such as for instance transformers, against potentially damaging over-voltage transients, usually surge arresters are employed. Such over-voltage transients may be caused both by external events, such as e.g. by lightning, or by internal events, such as e.g. resonances in a transformer winding induced by switching a circuit breaker connected to the transformer.
For safety purposes, surge arresters usually comprise a number of varistor blocks providing a conduction path for diverting and/or bypassing over-voltage transients safely to ground in case a varistor-dependent threshold in voltage is exceeded.
Generally, surge arresters are known in various embodiments. For instance WO 2011/095590 A1 and US 2012/0293905 A1 relate to a surge arrester with an active part and two electrodes arranged in a connecting element produced in an injection molding or die-casting process.
EP 0 642 141 A1 and U.S. Pat. No. 5,602,710 disclose a surge arrester with a varistor block between two connection fittings, which are cast with insulating material to form a monolithic body.
U.S. Pat. No. 4,604,673 discloses a shell-type distribution transformer with surge protection device comprising a metal oxide varistor device. In order to insulate the varistor device, it is mounted within an oil-filled tank. The varistor device is connected between a high-voltage winding of the transformer and ground. A further connection is provided between a mid-point of the varistor device and a mid-point of the high-voltage winding to protect the high voltage winding against both voltage surges entering via its terminals as well as current surges entering via terminals of a low voltage winding of the transformer. A drawback of such an integrated protection means may be that the complete final component cannot be tested with respect to its safe dielectric function and reliability, as described e.g. by standards, since the metal oxide varistor device may limit the test voltages.